Themes: Add a default image for when a theme doesn't have a screenshot. Props matveb. See #25959

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Themes: Remove some unused CSS (we have neither a .theme-categories or .theme-linear-grid). See #25948

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Themes: Update the responsive layouts for the Search box after r26461. See #26305

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Themes: Remove a now defunct CSS class, .feature-container, which is no longer in use after r26141. See #25948

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Themes: Remove the Tabs from the header of the Add New Theme page. Fixes #26310

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